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Abstract:

A polarization multiplexing transmitter which generates
polarization-multiplexed signals which are arbitrarily
polarization-scrambled at high speed, without adding a polarization
modulator and a polarization scrambler. In the transmitter, an
orthogonally polarized signal generator includes two optical modulators
which modulate the electric fields of optical signals and generate two
optical signals with mutually orthogonal polarized waves. The transmitter
includes electric field mappers which convert two data strings into
electric field signals, polarization mappers which give different
polarized waves to the two signals, polarization rotators which rotate
the polarized waves of the signals uniformly, a polarization multiplexer
which multiplexes the two polarization-rotated signals, a polarization
demultiplexer which demultiplexes the multiplexed signal into polarized
wave components of optical signals generated by the orthogonally
polarized signal generator, and a driver. The optical modulators are
driven to make the two demultiplexed electric field signals consistent
with the electric fields of optical signals modulated by the modulators.

Claims:

1. A polarization multiplexing transmitter comprising: an orthogonal
polarization multiplexing transmitter including: an orthogonally
polarized signal generator which generates two transmission electric
fields with mutually orthogonal polarized waves; two electric field
modulators which modulate amplitudes and/or phases of the two
transmission electric fields respectively; and a polarization multiplexer
which multiplexes two transmission electric field signals modulated by
the two electric field modulators and outputs a single
polarization-multiplexed signal; a polarization-rotated electric field
generator which outputs, for a plurality of data strings, a plurality of
polarization-rotated electric field signals having data on different
polarized waves rotating on a Poincare sphere and electric field data
corresponding to the data strings; and an electric field modulator driver
which multiplexes the polarization-rotated electric field signals into a
single multiplexed polarized electric field signal, demultiplexes the
multiplexed polarized electric field signal into polarized wave
components of the two transmission electric fields generated by the
orthogonally polarized signal generator and outputs two polarized
demultiplexed electric field signals, and drives the two electric field
modulators based on the two polarized demultiplexed electric field
signals respectively.

2. The polarization multiplexing transmitter according to claim 1,
wherein the two transmission electric fields generated by the
orthogonally polarized signal generator are two optical signals and the
two electric field modulators are two optical modulators which modulate
amplitudes and/or phases of the two optical signals respectively.

3. The polarization multiplexing transmitter according to claim 1,
wherein the electric field modulator driver includes: a plurality of
electric field mappers, with a function to convert data into electric
fields uniquely, which convert a plurality of data strings into electric
field signals respectively; a plurality of polarization mappers, with a
function to convert an incoming electric field into an arbitrary
polarization having that electric field, which convert the plural
electric field signals from the electric field mappers into polarized
electric field signals with desired polarized waves respectively; a
plurality of polarization rotators which rotate polarized waves of the
plural polarized electric field signals respectively, a polarization
rotation controller which controls the polarization rotators; a
polarization multiplexer which multiplexes the polarized electric field
signals from the polarization rotators into a multiplexed polarized
electric field signal; a polarization demultiplexer which demultiplexes
the multiplexed polarized electric field signal into two polarized wave
components of the two transmission electric fields generated by the
orthogonally polarized signal generator and outputs two polarized
demultiplexed electric field signals; and two drive signal generators
which drive the two electric field modulators respectively so that the
two polarized demultiplexed electric field signals from the polarization
demultiplexer are consistent with transmission electric field signals
from the two electric field modulators.

4. The polarization multiplexing transmitter according to claim 1,
wherein amplitudes and/or phases of all the polarization-rotated electric
field signals are each modulated independently and polarized waves of all
the polarization-rotated electric field signals are modulated on the
Poincare sphere uniformly.

5. The polarization multiplexing transmitter according to claim 4,
wherein the electric field modulator driver includes: a plurality of
electric field mappers, with a function to convert data into electric
fields uniquely, which convert a plurality of data strings into electric
field signals respectively; polarization mappers, with a function to
convert an incoming electric field into an arbitrary polarization having
that electric field, which convert the plural electric field signals into
a plurality of polarized electric field signals with desired polarized
waves respectively; a plurality of polarization rotators which rotate
polarized waves of the plural polarized electric field signals
respectively; a polarization rotation controller which synchronizes the
polarization rotators and drives the polarization rotators so as to
rotate incoming polarized waves uniformly and cyclically on the Poincare
sphere; a polarization multiplexer which multiplexes the polarized
electric field signals from the polarization rotators into a multiplexed
polarized electric field signal; a polarization demultiplexer which
demultiplexes the multiplexed polarized electric field signal into
polarized wave components of the two transmission electric fields
generated by the orthogonally polarized signal generator and outputs
polarized demultiplexed electric field signals; and two drive signal
generators which drive the two electric field modulators respectively so
that the two polarized demultiplexed electric field signals are
consistent with transmission electric field signals from the two electric
field modulators of the orthogonal polarization multiplexing transmitter.

6. The polarization multiplexing transmitter according to claim 4,
wherein the electric field modulator driver includes: a plurality of
electric field mappers, with a function to convert data into electric
fields uniquely, which convert a plurality of data strings into electric
field signals respectively; polarization mappers, with a function to
convert an incoming electric field into an arbitrary polarization having
that electric field, which convert the plural electric field signals into
a plurality of polarized electric field signals with desired polarized
waves respectively; a polarization multiplexer which multiplexes the
polarized electric field signals into a multiplexed polarized electric
field signal; a polarization rotator which rotates a polarized wave of
the multiplexed polarized electric field signal; a polarization rotation
controller which drives the polarization rotator so as to rotate an
incoming polarized wave cyclically; a polarization demultiplexer which
demultiplexes the multiplexed polarized electric field signal from the
polarization rotator into polarized wave components of the two
transmission electric fields generated by the orthogonally polarized
signal generator and outputs two polarized demultiplexed electric field
signals; and two drive signal generators which drive the two electric
field modulators respectively so that the two polarized demultiplexed
electric field signals are consistent with transmission electric field
signals from the two electric field modulators of the orthogonal
polarization multiplexing transmitter.

7. The polarization multiplexing transmitter according to claim 4,
wherein all of the cycles of polarized waves rotating of all the
polarization-rotated electric field signals are same each others; and the
least common of the cycles of polarized waves rotating and amplitudes
and/or phases modulation of all the polarization-rotated electric fields
is set to be smaller than a predetermined threshold.

8. The polarization multiplexing transmitter according to claim 4,
wherein the electric field modulator driver includes: a plurality of
electric field mappers, with a function to convert data into electric
fields uniquely, which convert a plurality of data strings into electric
field signals respectively; a plurality of polarization mappers, with a
function to convert an incoming electric field into an arbitrary
polarization having that electric field, which convert the plural
electric field signals into polarized electric field signals with
arbitrary polarizations on a circumference of a circle with an axis
connecting the polarized waves of the two transmission electric fields as
an axis of rotation on the Poincare sphere, a polarization multiplexer
which multiplexes the polarized electric field signals and outputs a
single multiplexed polarized electric field signal; a polarization
demultiplexer which demultiplexes the multiplexed polarized electric
field signal into polarized wave components of the two transmission
electric fields and outputs polarized demultiplexed transmission electric
field signals; two electric field modulation processors which modulate an
amplitude ratio and/or phase difference between the two polarized
demultiplexed electric field signals; an electric field modulation
controller which drives the electric field modulator; and two drive
signal generators which drive the two electric field modulators so that
the output fields from the two electric field modulation processors are
consistent with the two transmission electric fields modulated by the two
electric field modulators.

9. The polarization multiplexing transmitter according to claim 4,
wherein the electric field modulator driver includes: two electric field
mappers, with a function to convert data into electric fields uniquely,
which convert two data strings into electric field signals respectively;
electric field modulation processors which modulate a phase difference
between the two electric field signals; an electric field modulation
controller which drives the electric field modulation processors; and two
drive signal generators which drive the two electric field modulators
respectively so that the two electric field signals modulated by the
electric field modulation processors are consistent with the transmission
electric fields modulated by the electric field modulators.

10. The polarization multiplexing transmitter according to claim 4,
wherein a polarization rotation pattern is limited in rotation of
polarized waves of polarized electric field signals.

11. The polarization multiplexing transmitter according to claim 3,
wherein the two field modulation processors perform modulation so that a
group of electric field signals of incoming electric field signals are
consistent with a group of electric field symbols of outgoing electric
field signals.

12. The polarization multiplexing transmitter according to claim 1,
comprising: an electric field modulator driver including: a data string
alternation device which alternates two incoming data strings cyclically
and outputs two alternate data strings; two electric field mappers, with
a function to convert data into electric fields uniquely, which convert
the two alternate data strings into electric field signals respectively;
and two drive signal generators which drive the electric field modulators
so that the two electric field signals from the electric field mappers
are consistent with the transmission electric fields modulated by the
electric field modulators.

13. The polarization multiplexing transmitter according to claim 1,
comprising: an electric field modulator driver including: two electric
field mappers, with a function to convert data into electric fields
uniquely, which convert two incoming data strings into electric field
signals respectively; an electric field signal alternation device which
alternates two incoming electric field signals cyclically and outputs two
alternate electric field signals; and two drive signal generators which
drive the two electric field modulators so that the two alternate
electric field signals from the electric field signal alternation device
are consistent with the transmission electric fields modulated by the two
electric field modulators.

14. A transmission system comprising: a polarization multiplexing
transmitter according to claim 1; a transmission path polarization
monitor which detects a polarization fluctuation in a transmission path
from the polarization multiplexing transmitter to a receiver or its
residual, or an amount of dependence thereon; and a polarization
management device which drives the polarization multiplexing transmitter
based on the amount detected by the transmission path polarization
monitor, wherein the polarization multiplexing transmitter outputs a
polarization-multiplexed signal with a polarized wave so rotated as to
cancel a polarization fluctuation in the transmission path.

15. A transmission system comprising: a plurality of polarization
multiplexing transmitters according to claim 1; and at least one
polarization-uncontrolled transmitter which outputs a non-polarization
scrambled transmission signal, wherein the polarization multiplexing
transmitters and the polarization-uncontrolled transmitter include a
transmitting module with different wavelength channels and a polarization
scrambling management device which controls polarization scrambling
patterns and/or speeds of all the polarization multiplexing transmitters
so that in the different wavelength channels of all the polarization
multiplexing transmitters, polarization scrambling patterns and/or speeds
of the polarization multiplexing transmitters with adjacent wavelength
channels do not coincide with each other.

Description:

CLAIM OF PRIORITY

[0001] The present application claims priority from Japanese patent
application JP 2010-004146 filed on Jan. 12, 2010, the content of which
is hereby incorporated by reference into this application.

[0003] In recent optical transmission systems, the capacity of
transmission is increased by multilevel modulation. Multilevel modulation
is a technique which increases the capacity of transmission depending on
the number of modulation levels by modulating the amplitude and/or phase
of an optical signal into multiple levels. However, as the number of
modulation levels increases, the receiver sensitivity worsens, resulting
in a shorter transmission distance.

[0004] Therefore, next-generation optical transmission systems are
expected to increase the capacity of transmission by using not only a
multilevel modulation technique but also a polarization multiplexing
technique. Polarization multiplexing is a technique which combines
optical signals with different polarized waves (planes in which light
waves vibrate) to increase the capacity of transmission. Usually, two
optical signals whose polarized waves are orthogonal to each other are
combined to double the capacity of transmission. A polarization
multiplexing technique that uses two optical signals with mutually
orthogonal polarized waves is called orthogonal polarization
multiplexing. As described above, the use of polarization techniques is
attractive for next-generation large-capacity optical transmission
systems.

[0005] On the other hand, it is known that polarization causes various
problems related to signal degradation. For example, polarization
dependent loss (PDL) in optical waveguides, polarization dependent gain
(PDG) in optical amplifiers, and polarization hole burning (PHB) give
loss or gain to an optical signal depending on its polarization.
Furthermore, polarization mode dispersion (PMD) in optical fibers or
optical waveguides causes a delay in an optical signal depending on its
polarization.

[0006] The polarization dependence of an optical fiber is attributable to
the fact that stress on the optical fiber deforms its core cross section.
Such cross-sectional deformation of the core not only causes polarization
mode dispersion but also changes the polarized wave of the optical signal
depending on its wavelength and polarization. Also, it is known that
since generally the stress on the optical fiber is changing, the
polarization dependence of the optical fiber also changes with time.

[0007] These phenomena caused by polarization can seriously deteriorate
polarization multiplexed signal generated by polarization multiplexing.
For example, PDL causes different losses in multiplexed signals for
polarization multiplexed signal and also changes the polarization state
between multiplexed signals. This phenomenon is explained below referring
to FIGS. 1A to 1D. Let's suppose that a PDL device 001 transmits 100% of
optical power of a horizontally vibrating TE polarization and 33.3% of a
vertically vibrating TM polarization. When polarization multiplexed
signal as shown in FIG. 1A which combines an optical signal with a TE
polarization and one with a TM polarization enters the PDL device 001,
there is a difference in light intensity between the two multiplexed
polarized waves as shown in FIG. 1B. In this case, the polarization state
between multiplexed signals is maintained. On the other hand, when
polarization multiplexed signal as shown in FIG. 1C which combines an
optical signal with a 45°-rotated X polarization and one with a
45°-rotated Y polarization enters the PDL device 001, the
polarized waves of the two multiplexed signals rotate in opposite
directions and the angle between the polarized waves changes from 90
degrees to 120 degrees as shown in FIG. 1D. This phenomenon can be
interpreted to suggest that the X and Y polarizations as shown in FIG. 1C
are both a combination of TE and TM polarizations and their TM
polarization components are reduced by the PDL device 001 and
consequently they both become closer to the TE polarization. In fact,
when the PDL device 001 does not transmit the TM polarization, X and Y
polarizations both become the TE polarization.

[0008] Polarization of an optical signal can be visualized by a Poincare
sphere as shown in FIG. 2A. The Poincare sphere is a visualization tool
which uniquely represents a polarized wave 009 as a point on a spherical
surface. For example, FIG. 2A shows TE polarization 00A, TM polarization
00B, +45° polarized wave 0° C., -45° polarized wave
00D, right-handed circular polarized wave 00E, and left-handed circular
polarized wave 00F on the Poincare sphere. The line connecting the TE
polarization 00A and TM polarization 00B, the line connecting the
+45° polarized wave 0° C. and -45° polarized wave
00D, and the line connecting the right-handed circular polarized wave 00E
and left-handed circular polarized wave 00F are called S1, S2, and S3
axes respectively, in which these axes cross perpendicularly at the
center of the Poincare sphere. Polarized waves orthogonal to each other
like the TE polarization 00A and TM polarization 00B are expressed by
points located on opposite sides. Polarization dependence can be
understood to be a property that loss or delay varies depending on the
position of a point on the Poincare sphere.

[0009] Polarization scrambling is known as a technique to suppress signal
degradation caused by polarization dependence. Polarization scrambling is
a technique to change the polarization of an optical signal in order to
prevent the polarized wave of the signal from being fixed in a certain
state. Therefore, the polarized wave of a polarization-scrambled optical
signal has a temporal distribution 00H-1 as shown in FIG. 2B. Ideally,
the polarization of the optical signal should be changed so that the
optical signal polarized waves appear uniformly in a distribution 00H-2
covering the whole Poincare sphere as shown in FIG. 2C. Consequently the
polarization dependence of the optical signal is averaged, thereby
suppressing signal degradation caused by polarization dependence. Also,
as for mutually orthogonal polarized waves, their polarization
dependences are generally reverse, so it is effective to use a
polarization scrambler which rotates the waves cyclically on a
circumference with an axis passing through the center of the Poincare
sphere as the axis of rotation (for example, distribution 00H-3 shown in
FIG. 2D).

[0010]FIG. 3 shows a typical form of an optical transmission system which
uses a polarization scrambling technique. In this system, an optical
modulator (Mod) 003 modulates continuous light coming from a laser light
source (LD) 002 according to transmission data and outputs it as an
optical transmission signal. The polarized wave of the optical
transmission signal is constant. The optical transmission signal enters a
polarization scrambler (PS) 004 in which the polarized wave of the signal
is temporally rotated. This polarized wave rotation process is called
polarization scrambling. The optical transmission signal
polarization-scrambled by the polarization scrambler 004 enters an
optical fiber transmission path 005. Several optical repeaters (Nodes)
006 are inserted midway in the optical fiber transmission path 005 so as
to compensate for signal degradation caused by loss or wavelength
dispersion in the optical fiber transmission path 005. In some cases, an
optical repeater 006 has a polarization scrambler 004. Then, the optical
transmission signal which has entered the optical fiber transmission path
005 passes through the path 005 before it is received by an optical
receiver (Rx) 008. The optical receiver 008 demodulates the transmission
data in the received optical signal. If the polarization of the optical
signal which the optical receiver 008 receives is to be limited, a
polarization tracer (Pol. Tracer) 007 is inserted just before the optical
receiver 008 to eliminate a fluctuation in the polarization of the
optical signal so as to make the polarized wave of the optical signal
suitable for the optical receiver 008 in advance. Some types of optical
receiver 008 do not require an external polarization tracer 007 since
they incorporate a polarization tracing function.

[0011] The optical fiber transmission path 005 and optical repeater 006
have different types of polarization dependence because they include an
optical fiber, optical amplifier or optical waveguide type device. Since
the influence of such polarization dependence depends on the polarization
of the optical signal, if the polarization of the optical signal is
unchanged, a state in which signal degradation due to polarization
dependence is maximized may continue. Since polarization scrambling of
the optical transmission signal by the polarization scrambler 004 changes
the polarization of the optical signal, the influence of polarization
dependence of the optical fiber transmission path 005 can be averaged.
This effect can be enhanced when an error correction technique such as
forward error correction (FEC) is combined with polarization scrambling.
For example, FEC is a technique which divides the optical transmission
signal into FEC frames of several microseconds before transmission and
corrects an error of the received signal on a frame-by-frame basis. If
the optical signal is polarization-scrambled at a much higher speed (for
example, 10 MHz or more) than the FEC frame length, an error in the
received signal at the moment when the polarization of the optical signal
becomes the worst can be corrected using a received signal at another
time for the optical fiber transmission path 005 or optical repeater 006.
Also, since response to change in the polarization of the optical signal
is slow, at least PDG or PHB can be suppressed by polarization scrambling
at a much higher speed (for example, 100 kHz or more) than the response
speed.

[0012] Thus, polarization scrambling is an effective technique for
suppression of signal degradation caused by polarization dependence. As
described above, it is desirable to perform polarization scrambling at
high speed e. It is reported that signal degradation is more effectively
suppressed when polarization scrambling is synchronized with data
modulation and the polarization scrambling speed is made equal to the
data modulation speed, as described in Japanese Patent No. 3375811.

[0013] However, an ordinary polarization scrambler mechanically drives its
internal devices, so its polarization rotation speed is in the range from
several kilohertz to several megahertz. Not many polarization scramblers
are able to rotate polarized waves at higher speed with accuracy, though
electro-optical polarization scramblers are known to be able to rotate
polarized waves at a speed of 10 MHz or more.

[0014] A typical electro-optical polarization scrambler is a polarization
modulator which modulates the phase difference between two mutually
orthogonal TE and TM polarizations using an optical phase modulator. This
polarization modulator can rotate polarized waves on a circumference
00H-4 (FIG. 4A) with the S1 axis as the axis of rotation on the Poincare
sphere. The circumference 00H-4 includes a circumference 00G-3 which
passes through +45° polarized wave 0° C. and right-handed
circular polarized wave 00E, in which the +45° polarized wave
0° C. and right-handed circular polarized wave 00E can be
converted into each other. However, TE polarization 00A and TM
polarization 00B cannot be converted at all. Another problem is that even
the +45° polarized wave 0° C. and right-handed circular
polarized wave 00E cannot be converted, for example, into TE polarization
00A.

[0015] In this connection, a non-patent document authored by E. Hu et al.
and entitled "4-Level Direct-Detection Polarization Shift-Keying
(DD-PolSK) System with Phase Modulators" (OFC, FD2, 2003) reports a
polarization modulator which modulates the amplitude ratio and phase
difference between mutually orthogonal TE polarization 00A and TM
polarization 00B. This polarization modulator simulates polarization
rotation on a circumference 00H-5 (FIG. 4B) with the S3 axis as the axis
of rotation on the Poincare sphere by varying the amplitude ratio between
the TE polarization 00A component and TM polarization 00B component of an
incoming optical signal. Here, it is also possible to output an optical
signal with a desired polarized wave by modulating the phase difference
between the TE polarization 00A component and TM polarization 00B
component in addition to polarization rotation with the S1 axis as the
axis of rotation. However, an optical signal entering the polarization
modulator must have a TE polarization 00A component and a TM polarization
00B component. The polarization of an incoming signal with TE
polarization 00A component and TM polarization 00B component cannot be
converted arbitrarily. Desirably the incoming polarized wave should be a
+45° polarized wave 0° C. equally having the TE
polarization 00A component and TM polarization 00B component.

SUMMARY OF THE INVENTION

[0016] In an optical transmission system, generally the polarized wave of
an optical signal at the output end of the optical modulator 003 is fixed
so if a polarization rotator is installed there, the polarized wave can
be converted into a +45° polarized wave 0° C. and fixed.
Therefore, if the above polarization modulator is installed just after
the polarization rotator, the polarized wave of the optical transmission
signal can be changed arbitrarily. This kind of technique is described,
for example, in JP-A-2005-260696.

[0017] In any case, for polarization scrambling of an optical signal, it
is necessary to add a polarization modulator and its control circuit to
the optical transmitter, posing a problem of increase in equipment size
and cost. This problem is particularly serious for a transmission system
which uses many optical transmitters, such as a wavelength
demultiplexing/multiplexing (WDM) transmission system. As a solution to
this problem, JP-A-2004-253931 proposes a technique which integrates
polarization modulators for plural wavelength channels into one unit in a
WDM transmission system by installing, before a polarization modulator,
means to convert polarized waves into linearly polarizations such as TE
and TM polarizations. However, this technique also has a problem that the
polarization modulator must have a complicated structure and cannot be
used for polarization multiplexing.

[0018] In the field of wireless communications, JP-T-2007-506291 suggests
a technique which modulates two electric fields with mutually orthogonal
polarized waves adequately to generate a transmission signal with an
arbitrary polarization. This technique can be used to let the polarized
wave of a transmission signal hop according to transmission data or
multiplex arbitrarily polarized transmission signals.

[0019] This technique, an invention for wireless communications, has no
idea of polarization scrambling though it has an idea of polarized wave
hopping. The reason is that the polarization dependence of transmission
paths is small in wireless communications. In order to enable
polarization scrambling of an optical signal, along with the adoption of
the idea proposed in this patent document for an optical transmission
system, an optical signal polarization scrambling mechanism suitable for
the optical transmission system must be added. Particularly, a mechanism
for polarization scrambling of polarization-multiplexed signals is
anticipated in prospect of next-generation large-capacity optical
transmission systems.

[0020] An object of the present invention is to provide a polarization
scrambling optical transmitter which generates polarization-scrambled
modulated signals, particularly polarization-multiplexed signals without
newly adding a polarization modulator and a polarization scrambler.

[0021] According to a first aspect of the invention, there is provided a
polarization multiplexing transmitter which includes:

[0022] an orthogonal polarization multiplexing transmitter including an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate amplitudes and/or phases of the two
transmission electric fields respectively, and a polarization multiplexer
which multiplexes two transmission electric fields modulated by the two
electric field modulators and outputs a single polarization-multiplexed
signal; and

[0023] an electric field modulator driver which converts plural data
strings into plural electric field signals, coverts the electric field
signals into polarized electric field signals with arbitrary different
polarized waves, rotates the polarized waves of the polarized electric
field signals on a Poincare sphere, demultiplexes the polarized electric
field signal into electric field signals with two polarized wave
components consistent with the polarized waves of the two transmission
electric fields generated by the orthogonally polarized signal generator,
and drives the two electric field modulators based on the electric field
signals respectively.

[0024] According to a second aspect of the invention, there is provided a
polarization multiplexing transmitter which includes:

[0025] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two optical
signals with mutually orthogonal polarized waves, two optical modulators
which modulate the amplitudes and/or phases of the two optical signals,
and a polarization multiplexer which multiplexes two optical signals
modulated by the two optical modulators and outputs a single polarization
multiplexed signal; and

[0026] an optical modulator driver having plural electric field mappers,
with a function to convert data into electric fields uniquely, which
convert plural data strings into electric field signals respectively,
plural polarization mappers, with a function to convert an incoming
electric field into an arbitrary polarization having that electric field,
which convert the plural electric field signals from the plural electric
field mappers into polarized electric field signals with different
desired polarized waves respectively, a polarization multiplexer which
multiplexes the plural polarized electric field signals and generates a
multiplexed polarized electric field signal, a polarization demultiplexer
which demultiplexes the multiplexed polarization electric field signal
into electric field signals with two polarized wave components consistent
with the two optical signals generated by the orthogonally polarized
signal generator, and two drive signal generators which drive the two
optical modulators so that the electric field signals with two polarized
wave components from the polarization demultiplexer are consistent with
the electric fields of the optical signals coming from the two optical
modulators.

[0027] According to a third aspect of the invention, there is provided a
polarization multiplexing transmitter which includes:

[0028] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate the amplitudes and/or phases of the two
transmission electric fields, and a polarization multiplexer which
multiplexes two transmission electric fields from the orthogonally
polarized signal generator and outputs a single polarization-multiplexed
signal, in which the orthogonally polarized signal generator includes two
polarization electric field modulators which further modulate the
amplitudes and/or phases of the two transmission electric fields
modulated by the two electric field modulators and a polarization
electric field modulator driver which drives the two polarization
electric field modulators so as to modulate the amplitude ratio and/or
phase difference between the transmission electric fields modulated by
the two electric field modulators; and

[0029] plural electric field mappers, with a function to convert data into
electric fields uniquely, which convert plural data strings into electric
field signals respectively, polarization mappers, with a function to
convert an incoming electric field into an arbitrary polarization having
that electric field, which convert the plural electric field signals into
plural polarized electric field signals with different polarized waves as
arbitrary polarizations on a circumference of a circle with a line
connecting the polarized waves of the two transmission electric fields as
the axis of rotation on the Poincare sphere, a polarization multiplexer
which multiplexes the polarized electric field signals into a multiplexed
polarized electric field signal, a polarization demultiplexer which
demultiplexes the multiplexed polarized electric field signal into
electric field signals with two polarized wave components consistent with
the polarized waves of the two transmission electric fields, and two
drive signal generators which drive the two electric field modulators
respectively so that the electric field signals with two polarized wave
components from the polarization demultiplexer are consistent with the
two transmission electric fields modulated by the two electric field
modulators.

[0030] According to a fourth aspect of the invention, there is provided a
polarization multiplexing transmitter which includes:

[0031] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate the amplitudes and/or phases of the two
transmission electric fields, and a polarization multiplexer which
multiplexes two transmission electric fields emitted from the
orthogonally polarized signal generator and outputs a single
polarization-multiplexed signal; and

[0032] an electric field modulator driver having two electric field
mappers, with a function to convert data into electric fields uniquely,
which convert two data strings into electric field signals respectively,
an electric field phase modulator which modulates either or both of the
two electric field signals to modulate the phase difference between the
two electric field signals, and two drive signal generators which drive
the two electric field modulators respectively so that the two electric
field signals modulated by the electric field phase modulator are
consistent with the transmission electric fields modulated by the two
electric field modulators.

[0033] According to a fifth aspect of the invention, there is provided a
polarization multiplexing transmitter which includes:

[0034] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate the amplitudes and/or phases of the two
transmission electric fields respectively, and a polarization multiplexer
which multiplexes two transmission electric fields modulated by the two
electric field modulators and outputs a single polarization-multiplexed
signal; and

[0035] an electric field modulator driver having a data string alternation
device which alternates two incoming data strings cyclically and outputs
two alternate data strings, two electric field mappers, with a function
to convert data into electric fields uniquely, which convert the two
alternate data strings into electric field signals, and two drive signal
generators which drive the two electric field modulators respectively so
that the two electric field signals from the electric field mappers are
consistent with the electric fields modulated by the electric field
modulators.

[0036] According to a sixth aspect of the invention, there is provided a
polarization multiplexing transmitter which includes:

[0037] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate the amplitudes and/or phases of the two
transmission electric fields respectively, and a polarization multiplexer
which multiplexes two transmission electric fields modulated by the two
electric field modulators and outputs a single polarization-multiplexed
signal; and

[0038] an electric field modulator driver having two electric field
mappers, with a function to convert data into electric fields uniquely,
which convert two incoming data strings into electric field signals
respectively, an electric field signal alternation device which
alternates two incoming electric field signals cyclically and outputs two
alternate electric field signals, and two drive signal generators which
drive the two electric field modulators respectively so that two
alternate electric field signals from the electric field signal
alternation device are consistent with transmission electric fields
modulated by the two electric field modulators.

[0039] According to a seventh aspect of the invention, there is provided a
polarization multiplexing transmitter which includes:

[0040] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two optical
signals with mutually orthogonal polarized waves, two optical modulators
which modulate the amplitudes and/or phases of the two optical signals,
and a polarization multiplexer which multiplexes the two optical signals
from the two optical modulators and outputs a single polarization
multiplexed signal;

[0041] an optical modulator driver having two electric field mappers, with
a function to convert data into electric fields uniquely, which convert
two data strings into electric field signals respectively and two drive
signal generators which drive the two optical modulators respectively so
that the two electric field signals are consistent with the electric
fields of two optical signals from the two optical modulators;

[0042] a polarization modulator which modulates a phase difference between
two mutually orthogonal polarized wave components on a circumference of a
circle perpendicular to a line connecting two polarized waves from the
orthogonal polarization multiplexing transmitter on a Poincare sphere and
having the center of the Poincare sphere in its plane, and a driver which
drives the polarization modulator.

[0043] According to an eighth aspect of the invention, there is provided a
polarization multiplexing transmitter which includes:

[0044] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two optical
signals with mutually orthogonal polarized waves, two optical modulators
which modulate the amplitudes and/or phases of the two optical signals,
two polarization modulators which modulate the polarized waves of the two
optical signals respectively, and a polarization multiplexer which
multiplexes two optical signals modulated by the optical modulators and
polarization modulators and outputs a single polarization multiplexed
signal;

[0045] an optical modulator driver having two electric field mappers, with
a function to convert data into electric fields uniquely, which convert
two data strings into electric field signals respectively and two drive
signal generators which drive the two optical modulators so that the two
electric field signals are consistent with the electric fields of two
optical signals from the two optical modulators; and

[0046] a polarization modulator driver which drives the two polarization
modulators so that the angle between the polarized waves of output
signals from the two polarization modulators is maintained constant and
the polarized waves of the output signals from the two polarization
modulators are modulated uniformly.

[0047] According to a ninth aspect of the invention, there is provided a
transmission system which includes

[0048] a polarization multiplexing transmitter of any of the above types;

[0049] a transmission path polarization monitor which detects a
polarization fluctuation in a transmission path from the polarization
multiplexing transmitter to a receiver or its residual, or the amount of
dependence thereon; and

[0050] a polarization management device which drives the polarization
multiplexing transmitter based on the result of detection by the
transmission path polarization monitor,

[0051] in which the polarization multiplexing transmitter outputs a
polarization-multiplexed signal with a polarized wave so rotated as to
cancel a polarization fluctuation in the transmission path.

[0052] According to a tenth aspect of the invention, there is provided a
transmission system which includes:

[0053] a plurality of polarization multiplexing transmitters of any of the
above types and a plurality of polarization-uncontrolled transmitters
which output a non-polarization scrambled transmission signal, in which
the polarization multiplexing transmitters and the
polarization-uncontrolled transmitters include:

[0054] a transmitting module with different wavelength channels,

[0055] a multiplexer which multiplexes transmission signals from the
polarization multiplexing transmitters and polarization-uncontrolled
transmitters and transmits the multiplexed signal to a transmission path,
and

[0056] a polarization scrambling management device which controls the
polarization scrambling patterns and/or speeds of the polarization
multiplexing transmitters so that the polarization scrambling patterns
and/or speeds of the polarization multiplexing transmitters with adjacent
wavelength channels do not coincide with each other.

[0057] According to the present invention, using an orthogonal
polarization multiplexing transmitter, a polarization-scrambled optical
signal can be generated without newly adding a polarization modulator or
polarization scrambler to a transmitter. In particular,
polarization-scrambled polarization multiplexed signal can be generated.
In addition, according to the invention, since neither additional
polarization modulator nor polarization scrambler is required, the
equipment cost and size can be reduced.

[0058] Furthermore, according to the invention, the polarization
scrambling speed and/or pattern, cycle and timing of an optical signal
emitted from an optical transmitter can be as desired. Particularly, the
polarization scrambling speed can be a desired speed not higher than the
modulation speed of the optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] FIGS. 1A to 1D illustrate the phenomenon of signal degradation due
to PDL, in which FIG. 1A shows polarization-multiplexed light, FIG. 2B
shows a light intensity difference between the polarized waves of the
light,

[0062]FIG. 3 schematically shows an optical transmission system which
uses a polarization scrambling technique on the transmitting side;

[0063] FIGS. 4A to 4C illustrate operation of a polarization modulator, in
which FIG. 4A shows circumferences of the Poincare sphere for
polarization rotation, FIG. 4B shows polarization rotation with an axis
of rotation, and FIG. 4c shows polarization rotation with another axis of
rotation;

[0065] FIGS. 6A and 6B illustrate field mapping by QPSK, in which FIG. 6A
shows conversion of data strings into electric field signals and FIG. 6B
shows nine field symbols by quadrature amplitude modulation;

[0066] FIG. 7 shows an example of the configuration of a first embodiment
of the invention;

[0067] FIG. 8 shows an example of the configuration of a second embodiment
of the invention;

[0068] FIG. 9 shows an example of the configuration of a third embodiment
of the invention;

[0069]FIG. 10 shows an example of the configuration of a fourth
embodiment of the invention;

[0070] FIG. 11 shows an example of the configuration of a fifth embodiment
of the invention;

[0071] FIG. 12 shows an example of the configuration of a sixth embodiment
of the invention;

[0072]FIG. 13 shows an example of the configuration of a seventh
embodiment of the invention;

[0073] FIG. 14 shows an example of the configuration of an eleventh
embodiment of the invention;

[0074] FIG. 15 shows an example of the configuration of a twelfth
embodiment of the invention;

[0075] FIG. 16 shows an example of the configuration of a thirteenth
embodiment of the invention;

[0076] FIG. 17 shows an example of the configuration of a fourteenth
embodiment of the invention;

[0077]FIG. 18 shows an example of the configuration of an eighth
embodiment of the invention; and

[0078]FIG. 19 shows another example of the configuration of the eighth
embodiment of the invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0079] The preferred embodiments of the present invention use an
orthogonal polarization multiplexing transmitter which generates
polarization multiplexed signal.

[0080]FIG. 5 shows an example of the configuration of an orthogonal
polarization multiplexing transmitter.

[0081] The orthogonal polarization multiplexing transmitter has an
orthogonally polarized signal generator 010, a polarization multiplexer
012, and a modulator driver 013. The orthogonally polarized signal
generator 010 outputs two optical signals which have polarized waves
orthogonal to each other. The orthogonally polarized signal generator 010
has two optical modulators 011-1 and 011-2 which modulate the amplitudes
and/or phases of the optical signals with mutually orthogonal polarized
waves respectively. The polarization multiplexer 012 multiplexes or
combines the two polarized optical signals emitted from the optical
modulators 011-1 and 011-2 and outputs the multiplexed signal as
polarization multiplexed signal. The modulator driver 013 drives the two
optical modulators 011-1 and 011-2.

[0082] The orthogonally polarized signal generator 010 includes: a laser
light source 016 which outputs continuous light; an optical splitter 017
which bifurcates the continuous light into two optical waveguides; two
optical modulators 011-1 and 011-2 which modulate the amplitudes and/or
phases of the two continuous lights coming from the optical splitter 017
respectively; and a polarization orthogonalizer 018 which rotates one or
both of the polarized waves of the two optical signals modulated by the
optical modulators 011-1 and 011-2 and orthogonalizes the polarized
waves. The polarized orthogonalizer 018 is embodied in the form of a wave
plate, Faraday rotator or the like. However, the orthogonally polarized
signal generator 010 is not limited to the above configuration and may
employ any other appropriate means. An alternative configuration may be
that the optical splitter 017 is replaced by a polarization rotator to
rotate a polarized wave from the laser light source 016 to make it a
+45° polarized wave and a polarization demultiplexer to divide an
incoming optical signal into TE and TM polarizations and the polarization
orthogonalizer 010 is omitted.

[0083] The modulator driver 013 converts two different data strings into
electric field signals respectively using two electric field mappers
014-1 and 014-2 with a function to convert data into electric fields
uniquely. Two drive signal generators 015-1 and 015-2 output drive
signals to the optical modulators 011-1 and 011-2 respectively so that
these electric field signals are consistent with the electric fields of
the optical signals coming from the optical modulators 011-1 and 011-2.
This is just an example of the configuration of the modulator driver 013
and any other configuration may be adopted as far as it can drive the
optical modulators 011-1 and 011-2 according to transmission data.

[0084] For the conversion of data and electric fields in the electric
field mappers 014-1 and 014-2, various modulation methods for converting
the amplitudes and/or phases of electric fields are available. For
example, if the quadrature phase shift keying (QPSK) method is selected,
electric field mapping as shown in FIG. 6A is applied in which 2-bit data
00, 01, 10, and 11 are converted into electric field symbols with
different phases, 019-1, 019-2, 019-3, and 019-4.

[0085] Generally the optical modulators 011-1 and 011-2 used in this
orthogonal polarization multiplexing transmitter are electro-optical
modulators. Therefore, when the orthogonal polarization multiplexing
transmitter is embodied in the following forms, polarized waves of output
signals can be scrambled at high speed.

First Embodiment

[0086] As explained earlier, in an ordinary electro-optical polarization
scrambler, not only polarizations which can be emitted but also the
polarized wave of incoming light are limited. Polarization multiplexed
signal from an orthogonal polarization multiplexing transmitter is a
signal obtained by multiplexing different polarized waves and it is
difficult to bring these polarized waves into a specific polarized state
simultaneously.

[0087] Therefore, a possible approach is to limit the polarized waves of a
multiplexed signal as polarization multiplexed signal to polarized waves
which can be rotated by an electro-optical polarization scrambler. This
will make polarization-scrambling of polarization multiplexed signal
possible.

[0088] However, some kinds of polarized waves can be rotated most
effectively by an electro-optical polarization scrambler.

[0089] For example, a polarization scrambler having a polarization
modulator which modulates the phase difference between two mutually
orthogonal polarized waves (for example, +45° polarized wave
0° C. and -45° polarized wave 00D) performs polarization
rotation of the optical signal on a circumference (circumference 00H-6 in
FIG. 4c in the example) having, as the axis of rotation, the line (S2
axis in the example) connecting the two polarized waves to be modulated,
on the Poincare sphere. Although the electro-optical polarization
scrambler cannot perform considerable polarization rotation of even
incoming signals with polarized waves similar to the polarized waves to
be modulated on the Poincare sphere (+45° polarized wave 0°
C. and -45° polarized wave 00D in the example), it can perform
considerable polarization rotation on incoming signals with polarized
waves whose coordinates are orthogonal to each other on the Poincare
sphere (TE polarization 00A, TM polarization 00B, right-handed circular
polarized wave 00E, and left-handed circular polarized wave 00F in the
example).

[0090] Therefore, in this embodiment, the polarized waves of signals to be
multiplexed by the orthogonal polarization multiplexing transmitter are
polarized waves which an electro-optical polarization scrambler can
rotate most effectively.

[0091] FIG. 7 shows an example of the configuration of the polarization
multiplexing optical transmitter according to the first embodiment.

[0092] The optical polarization multiplexing transmitter includes: an
orthogonally polarized signal generator 010 which modulates the
amplitudes and/or phases of optical signals with TE and TM polarizations
according to two data strings respectively and output the modulated
waves; a polarization multiplexer 012 which multiplexes the two optical
signals emitted from the orthogonal polarization multiplexing transmitter
011 and outputs a single polarization multiplexed signal; a polarization
modulator 020 which modulates the phase difference between the
+45° polarized wave component and -45° polarized wave
component of the polarization multiplexed signal; a polarization
modulation driver 021 which generates a drive signal for the polarization
modulator 020; and a modulator driver 013. In this specification, the
orthogonally polarized signal generator 010 and polarization multiplexer
012 are sometimes collectively called an orthogonal polarization
multiplexing transmitter.

[0093] The orthogonally polarized signal generator 010 may include: a
laser light source 016 which emits continuous light with TE
polarizations; an optical splitter 017 which bifurcates the continuous
light; optical modulators 011-1 and 011-2 which modulate the amplitudes
and/or phases of the two continuous lights coming from the optical
splitter 017 respectively; and a polarization orthogonalizer 018 which
converts the polarized waves of the optical signals coming from the
optical modulators 011-1 and 011-2 into TE and TM polarizations.

[0094] The optical modulators 011-1 and 011-2 are driven by the modulator
driver 013. The configuration of the modulator driver 013 is the same as
that of the modulator driver 013 of the orthogonal polarization
multiplexing transmitter shown in FIG. 5.

[0095] The polarization modulator 020 performs polarization rotation of
the optical signal on circumference 00H-6 (FIG. 4c) having, as the axis
of rotation, the S2 axis connecting the +45° polarized wave
0° C. and -45° polarized wave 00D on the Poincare sphere.
This polarization rotation largely changes the polarized waves on equator
00G-2 (FIG. 4c) with the S2 axis as the axis of rotation on the Poincare
sphere. Thus, the TE polarization 00A and TM polarization 00B which are
multiplexed by the orthogonal polarization multiplexing transmitter can
be largely rotated. The polarized waves of signals multiplexed by the
orthogonal polarization multiplexing transmitter should be polarized
waves on the circumference 00G-2 (FIG. 4c) and may be a right-handed
circular polarized wave 00E or left-handed circular polarized wave
0° F. The optical polarization multiplexing transmitter thus
configured emits polarization multiplexed signal which rotates uniformly
on the circumference 00G-2 of the Poincare sphere (FIG. 4c). Although in
this example the polarization modulator which modulates the phase
difference between two mutually orthogonal polarized wave components is
used as an electro-optical polarization scrambler, it is also possible to
use another type of polarization modulator. For example, if a
polarization modulator which modulates the amplitude ratio between TE and
TM polarizations is used as the polarization modulator 020, the polarized
waves of signals which are multiplexed by the polarization multiplexing
transmitter 010 should be polarized waves on the circumference 00G-3
(FIG. 4A), for example, +45° polarized waves. The reason is that
if the polarization modulator 020 converts an optical signal with a small
TM polarization component into a TM polarization, the intensity of output
light is low, so desirably incoming light should have a TM polarization
component and a TE polarization component equally.

[0096] In this example, polarization multiplexed signal with various
polarized waves can be polarization-scrambled by varying the combination
of polarized waves multiplexed by the polarization multiplexing
transmitter 010 and polarized waves modulated by the polarization
modulator 020.

Second Embodiment

[0097] If optical signals are not polarization-multiplexed yet, the
polarized waves of the optical signals can be converted into polarized
waves suitable for an electro-optical polarization scrambler. Thus,
polarization-scrambled polarization multiplexed signal can be generated
by multiplexing the polarized waves of the optical signals after they are
modulated by an electro-optical polarization scrambler.

[0098] FIG. 8 shows an example of the configuration of the second
embodiment.

[0100] The orthogonally polarized signal generator 010 may include: a
laser light source 016 which emits continuous light with TE
polarizations; an optical splitter 017 which bifurcates the continuous
light; optical modulators 011-1 and 011-2 which modulate the amplitudes
and/or phases of the two continuous lights coming from the optical
splitter 017 respectively; two polarization rotators 028 which convert
the polarized waves of the optical signals coming from the optical
modulators 011-1 and 011-2 into, for example, +45° polarized waves
respectively; and two polarization modulators 020-1 and 020-2 which
modulate the phase difference and/or amplitude ratio between the TE and
TM polarization components of each of the optical signals coming from the
two polarization rotators 028.

[0101] The optical modulators 011-1 and 011-2 are driven by the modulator
driver 013. The configuration of the modulator driver 013 is the same as
that of the modulator driver 013 of the orthogonal polarization
multiplexing transmitter shown in FIG. 5.

[0102] The two polarization modulators 020-1 and 020-2 are driven by the
polarization modulation driver 021 and their output polarized waves are
controlled. For example, if polarization multiplexed signal from the
polarization multiplexer 012 is to be an orthogonally polarized
multiplexed signal, the two polarization modulators 020-1 and 020-2 are
driven so that their output polarized waves become orthogonal to each
other. Also, if optical signals from the optical modulators 011-1 and
011-2 are to be multiplexed with different polarized waves and the
resulting polarization-multiplexed signal light is to be
polarization-scrambled and emitted, the polarization modulators 020-1 and
020-2 are driven so that the output polarized waves from the polarization
modulators 020-1 and 020-2 are different from each other and while their
relation is maintained, they rotate uniformly on the Poincare sphere.

[0103] For the polarization modulators 020-1 and 020-2, various types of
electro-optical polarization modulators are available. For example, the
electro-optical polarization scrambler described earlier in the section
of the Background of the Invention may be used. By using a polarization
modulator capable of converting the polarized waves of output signals
arbitrarily, it is also possible to generate an optical signal whose
polarized wave changes uniformly all over the Poincare sphere. The
optical signal polarization distribution generated in this case is shown
as distribution 00H-2 in FIG. 2C.

[0104] Although optical signals with two different polarized waves are
multiplexed in this example, it is also possible to multiplex more than
two polarized optical signals. It should be however noted that more
optical modulators, polarization rotators, and polarization modulators
are required depending on the number of signals to be multiplexed.

Third Embodiment

[0105] The orthogonal polarization multiplexing transmitter shown in FIG.
5 includes the polarization modulator described in the aforementioned
non-patent document authored by E. Hu et al. and can be used as a
polarization modulator. This suggests that a single orthogonal
polarization multiplexing transmitter can be used to perform polarization
modulation and electric-field modulation based on data simultaneously. In
the field of wireless communications, this kind of technique is disclosed
in JP-T-2007-506291.

[0106] In this embodiment, an optical polarization-multiplexed signal with
an arbitrarily polarization is generated by modulating the amplitudes
and/or phases of two optical signals with mutually orthogonal polarized
waves adequately.

[0107] FIG. 9 shows an example of the configuration of the third
embodiment.

[0108] The optical polarization multiplexing transmitter includes: an
orthogonally polarized signal generator 010 which has two optical
modulators 011-1 and 011-2 for modulating the amplitudes and/or phases of
two optical signals respectively and generates optical signals with two
mutually orthogonal polarized waves (for example, TE and TM
polarizations); a polarization multiplexer 012 which multiplexes the two
optical signals modulated by the two optical modulators 011-1 and 011-2
and outputs a single polarization multiplexed signal; and a modulator
driver 013 which drives the two optical modulators 011-1 and 011-2. This
general configuration is the same as that of the orthogonal polarization
multiplexing transmitter shown in FIG. 5, but the internal configuration
of the modulator driver 013 is largely different.

[0109] In the modulator driver 013, two electric field mappers 014-1 and
014-2 with a function to convert data into electric fields uniquely
convert two data strings into electric field signals respectively. Two
polarization mappers 022-1 and 022-2 with a function to convert an
incoming electric field into an arbitrary polarization having that
electric field convert the two electric field signals into polarized
electric field signals with different polarized waves as desired (for
example, a TE polarization and a 45° polarized wave) respectively.
A polarization multiplexer 023 multiplexes the two polarized electric
field signals into a multiplexed polarized electric field signal. A
polarization demultiplexer 024 demultiplexes the multiplexed polarized
electric field signal into two polarized wave components consistent with
the polarized waves (for example, TE and TM polarizations) of the two
optical signals generated by the orthogonally polarized signal generator
010. Two drive signal generators 015-1 and 015-2 generate drive signals
to drive the optical modulators 011-1 and 011-2 so that the electric
field signals with the two polarized wave components from the
polarization demultiplexer 024 are consistent with the electric fields of
the optical signals coming from the two optical modulators 011-1 and
011-2. Since the optical transmitter uses the optical modulators 011-1
and 011-2 with non-linear input/output characteristics to modulate
optical signals, the drive signal generators 015-1 and 015-2 are used to
cancel such input/output characteristics.

[0110] In the example shown in FIG. 9, two data strings are converted into
two polarized electric field signals, but the number of data strings to
be converted into polarized electric field signals can be an arbitrary
number N. However, if that is the case, it is necessary to use N electric
field mappers which convert N data strings into electric field signals
respectively, N polarization mappers which convert the N electric field
signals into polarized electric field signals with arbitrary different
polarized waves respectively, and a polarization multiplexer 023 which
multiplexes the N polarized electric field signals into a multiplexed
polarized electric field signal.

[0111] The modulator driver 013 includes polarization mappers 022-1 and
022-2 and polarization processors such as a polarization multiplexer 023
and a polarization demultiplexer 024 so as to process the polarized waves
of multilevel-modulated electric field signals.

[0112] An electric field signal with a polarized wave can be represented
by a Stokes vector or Jones vector. A Stokes vector is a
three-dimensional vector composed of electric fields of the S1, S2, and
S3 axes of the Poincare sphere shown in FIG. 2A, while a Jones vector is
a two-dimensional vector composed of electric fields Ex and Ey
of mutually orthogonal polarized waves, such as TE polarization 00A and
TM polarization 00B, as expressed by Equation 1.

( E x E y ) = ( I x + j Q x
I y + j Q y ) [ Equation 1 ]
##EQU00001##

[0113] Here, Ix and Qx denote the real part and imaginary part
of electric field Ex as expressed on a complex plane; the square
root of Ix2+Qx2 denotes the electric field amplitude
of Ex; and tan-1 (Qx, Ix) denotes the electric field
phase of E. Likewise, Iy and Qy denote the real part and
imaginary part of electric field Ey. Although processing of
polarized waves is explained using Jones vectors in this specification,
polarized waves may be processed using another expression method such as
Stokes vectors.

[0114] The polarization mappers 022-1 and 022-2 may be considered to
convert incoming electric field signals into

[0115] Jones vectors. For example, the Jones vector of right-handed
circular polarized wave 00E is expressed by Equation 2 and for the
conversion of electric field Eo into a right-handed circular
polarized wave, a Jones vector expressed by Equation 3 is created by
multiplication of both.

[0116] The polarization multiplexer 023 can be considered to sum up a
plurality of incoming Jones vectors. The polarization demultiplexer 024
can be considered to divide an incoming Jones vector into components and
output them. However, in order to divide a Jones vector with TE
polarization 00A and TM polarization 00B into polarized waves different
from them (for example, +45° polarized wave 0° C. and
-45° polarized wave 00D), the vector must be converted into a
Jones vector with desired polarized wave components before it is divided.
Conversion of polarized waves can be represented by a Jones matrix of two
rows and two columns. Conversion of polarized waves will be explained
next when the fourth embodiment is described.

[0117] The above electric field mappers may output mapped electric fields
in a linearly distorted form (such as wavelength dispersion). For this
purpose, a preliminary wavelength dispersion technique may be combined.

Fourth Embodiment

[0118] In the third embodiment, optical polarization-multiplexed signals
whose polarized waves are modulated arbitrarily can be generated by
modulating the polarized waves of polarized electric field signals before
they are multiplexed by the polarization multiplexer 023.

[0119]FIG. 10 shows an example of the configuration of the fourth
embodiment.

[0120] Here, newly added to the configuration of the third embodiment
shown in FIG. 9 are: polarization rotators 025-1 and 025-2 which rotate
the polarized waves of two polarized electric field signals coming from
the polarization mappers 022-1 and 022-2 respectively; and polarization
rotation controllers 026-1 and 026-2 which drive the polarization
rotators 025-1 and 025-2 respectively. The polarization multiplexer 023
multiplexes the two polarized electric field signals rotated by the
polarization rotators 025-1 and 025-2 and outputs a single multiplexed
polarized electric field signal.

[0121] In this embodiment as well, the number of data strings to be
converted into polarized electric field signals can be an arbitrary
number N. However, if that is the case, it is necessary to use N electric
field mappers which convert N data strings into electric field signals
respectively, N polarization mappers which convert the N electric field
signals into polarized electric field signals, N polarization rotators
which rotate the polarized waves of the N polarized electric field
signals, N polarization rotation controllers which drive the N
polarization rotators, and a polarization multiplexer 023 which
multiplexes the N polarized electric field signals into a multiplexed
polarized electric field signal.

[0122] In this embodiment, the polarization rotators 025-1 and 025-2
modulate the polarized waves of polarized electric field signals coming
from the polarization mappers 022-1 and 022-2 respectively. Therefore,
the polarized waves of polarized electric field signals from the
polarization mappers 022-1 and 022-2 may coincide with each other.
However, in that case, it is necessary to give an offset to polarization
rotation by either the polarization rotator 025-1 or 025-2 so that the
relation between the polarized waves of the polarized electric field
signals is adequate.

[0123] Rotation of polarization expressed by a Jones vector can be
represented by the product of the Jones vector and a Jones matrix of two
rows and two columns as expressed by Equation 4.

( R 00 R 01 R 10 R 11 ) [ Equation
4 ] ##EQU00003##

[0124] Here, each component of the Jones matrix is expressed by a function
of two parameters θ and φ, which are identical to the angles
θ and φ representing an arbitrary polarization 009 on the
Poincare sphere shown in FIG. 4A. A simple example of a Jones matrix to
rotate an incoming polarized wave by angle θ is expressed below by
Equation 5.

( cos θ - sin θ sin
θ cos θ ) [ Equation 5 ]
##EQU00004##

[0125] When the polarized wave represented by the Jones vector of Equation
3 is rotated by angle θ, the rotated polarized wave is represented
by a Jones vector of Equation 6 as shown below.

[0126] The same is true for angle φ. Thus, the polarization rotators
025-1 and 025-2 can be interpreted to multiply an incoming Jones vector
by a Jones matrix.

Fifth Embodiment

[0127] This embodiment is a variation of the fourth embodiment which
generates optical polarization-multiplexed signals which are
polarization-scrambled arbitrarily.

[0128] FIG. 11 shows an example of the configuration of the fifth
embodiment.

[0129] As compared with the configuration of the fourth embodiment shown
in FIG. 10, this embodiment newly adds one polarization synchronization
controller 027 for driving the polarization rotators 025-1 and 025-2, in
place of the two polarization rotation controllers 026-1 and 026-2. The
polarization synchronization controller 027 synchronizes the polarization
rotators 025-1 and 025-2 and drives them so that they uniformly rotate
the polarized waves of signals entering them. Consequently, the polarized
waves of two polarized electric field signals are rotated while the
relation between these waves is maintained, so that the polarization
multiplexer 012 emits polarization multiplexed signal with polarized
waves rotated in the same way. It is also possible to rotate polarized
waves cyclically or in a specific pattern. It is also possible to rotate
the polarized waves of the two polarized electric field signals without
maintaining the relation between the polarized waves, or rotate the
polarized waves independently from each other.

[0130] Any means may be adopted to synchronize the two polarization
rotators 025-1 and 025-2. One method for such synchronization is that the
polarization synchronization controller 027 sends an equal drive signal
to the two polarization rotators 025-1 and 025-2 and uses the
transmission paths of the same length for the signal. Another method is
that the relation between the polarized waves of the two polarized
electric field signals rotated by the two polarization rotators 025-1 and
025-2 is detected at the input end of the polarization multiplexer 023.

[0131] In this embodiment as well, the number of data strings to be
converted into polarized electric field signals can be an arbitrary
number N. However, if that is the case, it is necessary to use N electric
field mappers which convert N data strings into electric field signals
respectively, N polarization mappers which convert the N electric field
signals into polarized electric field signals, N polarization rotators
which modulate the N polarized electric field signals, a polarization
synchronization controller 027 which drives the N polarization rotators,
and a polarization multiplexer 023 which multiplexes the N polarized
electric field signals into a multiplexed polarized electric field
signal.

[0132] In this embodiment as well, the polarization rotators 025-1 and
025-2 modulate the polarized waves of polarized electric field signals
coming from the polarization mappers 022-1 and 022-2 respectively.
Therefore, the polarized waves of polarized electric field signals from
the polarization mappers 022-1 and 022-2 may coincide with each other.
However, in that case, it is necessary to give an offset to polarization
rotation by either the polarization rotator 025-1 or 025-2 so that the
relation between the polarized waves of the polarized electric field
signals is adequate.

[0133] This embodiment can be applied to telecommunications and wireless
communications in which electricity and radio waves are used for
transmission signals. In that case, the laser light source (LD) should be
a device which generates electric signals or radio signals.

Sixth Embodiment

[0134] In the fifth embodiment, plural polarization rotators must be
synchronized. In large-capacity optical transmissions, the signal speed
may exceed 20 gigabits/second and if the synchronization error tolerance
is, for example, 1/10, synchronization must be controlled with an
accuracy of 5 picoseconds (equivalent to 1 millimeter light path) or
less. In addition, the number of polarization rotators must be increased
depending on the number (N) of data strings or polarized waves to be
transmitted.

[0135] Therefore, in this embodiment, one polarization rotator is used
instead of the above plural polarization rotators and the polarization
synchronization controller 027 is eliminated.

[0136] FIG. 12 shows an example of the configuration of the sixth
embodiment.

[0137] As compared with the configuration of the fifth embodiment shown in
FIG. 11, this embodiment eliminates the polarization rotators 025-1 and
025-2 and the polarization synchronization controller 027 which drives
them. The polarized electric field signals coming from the two
polarization mappers 022-1 and 022-2 are multiplexed into a single
polarized electric field signal by the polarization multiplexer 023.
Also, this embodiment newly adds a polarization rotator 025-3 which
rotates the polarized waves of the multiplexed polarized electric field
signal and outputs it to the polarization demultiplexer 024 and a
polarization rotation controller 026-3 which drives the polarization
rotator 025-3. Consequently, the polarized waves of two polarized
electric field signals from the polarization mappers 022-1 and 022-2 are
uniformly rotated while the relation between the polarized waves is
maintained, so that the polarization multiplexer 012 emits polarization
multiplexed signal with polarized waves rotated in the same way. The
polarization rotation controller 026-3 controls the pattern and
periodicity of polarization rotation.

[0138] In this embodiment as well, the number of data strings to be
converted into polarized electric field signals can be an arbitrary
number N. However, if that is the case, it is necessary to use N electric
field mappers which convert N data strings into electric field signals
respectively, N polarization mappers which convert the N electric field
signals into polarized electric field signals, and a polarization
multiplexer 023 which multiplexes the N polarized electric field signals
into a multiplexed polarized electric field signal.

[0139] This embodiment can also be applied to telecommunications and
wireless communications in which electricity and radio waves are used for
transmission signals.

Seventh Embodiment

[0140] As described in the aforementioned non-patent document authored by
E. Hu et al, by dividing one polarized wave into two mutually orthogonal
polarized wave components and modulating the amplitude ratio and phase
difference between them, the polarization can be modulated. The seventh
embodiment uses a simplified form of the modulator driver 013 based on
this kind of polarization modulation.

[0141]FIG. 13 shows an example of the configuration of the seventh
embodiment.

[0142] The polarization multiplexing transmitter includes: an orthogonal
polarization multiplexing transmitter including an orthogonally polarized
signal generator 010 which has two optical modulators 011-1 and 011-2 for
modulating the amplitudes and/or phases of two optical signals and
generates optical signals with two mutually orthogonal polarized waves
(for example, TE polarization 00A and TM polarization 00B) and a
polarization multiplexer 012 which multiplexes the two optical signals
modulated by the two optical modulators 011-1 and 011-2 and outputs a
single polarization multiplexed signal; and a modulator driver 013 which
drives the optical modulators 011-1 and 011-2.

[0143] In the modulator driver 013, two electric field mappers 014-1 and
014-2 with a function to convert data into electric fields uniquely
convert two data strings into electric field signals and output the
signals to the polarization mappers 022-1 and 022-2 respectively. The
polarization mappers 022-1 and 022-2 have a function to convert an
incoming electric field into an arbitrary polarization having that
electric field and convert the two electric field signals into polarized
electric field signals with specific polarized waves. The specific
polarized waves are arbitrary polarizations (+45° polarized wave
0° C. and -45° polarized wave 00D in the example) on an
equator (for example, circumference 00G-3 in FIG. 4A) with the line (S1
axis in the example) connecting two polarized waves generated by the
orthogonally polarized signal generator 010 as the axis of rotation on
the Poincare sphere. A polarization multiplexer 023 multiplexes the two
polarized electric field signals into a multiplexed polarized electric
field signal. After a polarization demultiplexer 024 demultiplexes the
multiplexed polarized electric field signal into two polarized wave
components (TE polarization 00A and TM polarization 00B in the example)
generated by the orthogonally polarized signal generator 010, the
electric field modulation processors 029-1 and 029-2 modulate the
amplitudes and/or phases of the polarized wave components. The electric
field modulation processors 029-1 and 029-2 are synchronized and driven
by a field modulation controller 030. Then, two drive signal generators
015-1 and 015-2 drive the optical modulators 011-1 and 012 respectively
so that the two electric field signals emitted from the electric field
modulation processors 029-1 and 029-2 are consistent with the electric
fields of optical signals emitted from the optical modulators 011-1 and
011-2.

[0144] Thus, polarization multiplexed signal which is obtained by
multiplexing optical signals with polarized waves (+45° polarized
wave 0° C. and -45° polarized wave 00D in the example)
generated by the polarization mappers 022-1 and 022-2 is
polarization-scrambled and the polarization-scrambled
polarization-multiplexed light is emitted from the polarization
multiplexer 012. In this embodiment, optical signals which do not have
two polarized waves (TE polarization 00A and TM polarization 00B in the
example) generated by the orthogonally polarized signal generator 010 are
multiplexed and the multiplexed signal light is emitted.

[0145] This embodiment can be considered to combine a first polarization
modulator and a second polarization modulator in which the former
divides, for example, a +45° polarized wave into TE and TM
polarization components and modulates the phase ratio and/or phase
difference between them and the latter divides a -45° polarized
wave into TE and TM polarization components and modulates the phase ratio
and/or phase difference between them.

[0146] In this embodiment as well, the number of data strings to be
converted into polarized electric field signals can be an arbitrary
number N. However, if that is the case, it is necessary to use N electric
field mappers which convert N data strings into electric field signals
respectively, N polarization mappers which convert the N electric field
signals into polarized electric field signals, and a polarization
multiplexer 023 which multiplexes the N polarized electric field signals
into a multiplexed polarized electric field signal.

[0147] The steps from the electric field mappers 014-1 and 014-2 to the
polarization multiplexer 023 may also be implemented in another form. For
example, if the electric field mappers 014-1 and 014-2 convert data
strings into electric field signals using QPSK shown in FIG. 6A, the
polarization multiplexer 023 outputs an electric field symbol among the
nine electric field symbols of the QAM (Quadrature Amplitude Modulation)
shown in FIG. 6B. This is because the two components of a Jones vector
from the polarization multiplexer 023 turn into an electric field signal
multiplexed with an electric field signal obtained by multiplexing the
two electric field signals generated by the electric field mappers 014-1
and 014-2 with a phase difference of 180 degrees between them. Therefore,
the steps from the electric field mappers 014-1 and 014-2 to the
polarization multiplexer 023 may be implemented by logical operations of
data strings based on the abovementioned fact.

[0148] When the electric field modulation processors 029-1 and 029-2
modulate the amplitude ratio between two mutually orthogonal polarized
wave components (for example, TE and TM polarizations), the relation
between the two polarized wave components is lost as in the case of
signal degradation due to PDL as described in reference to FIGS. 1A to
1D. For this reason, it is acceptable that the electric field modulation
processors 029-1 and 029-2 modulate only the phase difference between the
two polarized wave components. Simply by doing so, polarization rotation
can be performed on the multiplexed polarized electric field signal
emitted from the polarization multiplexer 023 with the line (S1 axis in
the example) connecting the two polarized wave components as the axis of
rotation on the Poincare sphere. If one data string is assigned to one
polarized wave for transmission or the relation between the two polarized
wave components need not be maintained, the electric field modulation
processors 029-1 and 029-2 may be used to modulate the amplitude ratio
between the two polarized waves.

[0149] This embodiment can be applied to telecommunications and wireless
communications in which electricity and radio waves are used for
transmission signals.

Eighth Embodiment

[0150] Even an orthogonally polarized multiplexed signal which combines
two mutually orthogonal polarized waves electric-field-modulated
independently from each other is observed on the Poincare sphere as a
single state of light with two different polarized waves. This
polarization varies depending on the amplitude ratio and phase difference
between the two polarized waves.

[0151] In the eighth embodiment, the polarization of an orthogonally
polarized multiplexed signal is modulated by modulating the phase
difference between two mutually orthogonal polarized waves
electric-field-modulated independently.

[0152]FIG. 18 shows an example of the configuration of the eighth
embodiment. The configuration in this example is explained below.

[0153] The configuration includes: an orthogonal polarization multiplexing
transmitter having an orthogonally polarized signal generator 010 which
generates optical signals with two mutually orthogonal polarized waves,
two optical modulators 011-1 and 011-2 which modulate the amplitudes
and/or phases of the two optical signals, and a polarization multiplexer
012 which multiplexes the two optical signals modulated by the two
optical modulators 011-1 and 011-2 and outputs a single polarization
multiplexed signal; and a modulator driver 013 which drives the two
optical modulators 011-1 and 011-2.

[0154] The modulator driver 013 converts two data strings into electric
field signals using two electric field mappers 014-1 and 014-2 with a
function to convert data into electric fields uniquely and includes
electric field phase modulators 043-1 and 043-2 to modulate the phase(s)
of either or both of the two electric field signals in order to modulate
the phase difference between the two electric field signals, and drives
the optical modulators 011-1 and 011-2 respectively so that the two
electric field signals modulated by the electric field phase modulators
043-1 and 043-2 are consistent with the electric fields of optical
signals emitted from the optical modulators 011-1 and 011-2.

[0155] The electric field phase modulators 043-1 and 043-2 can modulate
the phase difference between two electric field signals emitted from the
mappers 014-1 and 014-2 in a desired pattern such as a sinusoidal wave
pattern.

[0156]FIG. 19 is another example of the configuration of the eighth
embodiment. Inter-polarization phase difference modulators 044-1 and
044-2 which modulate the phase(s) of either or both of the two optical
signals multiplexed by the polarization multiplexer 012 may be used in
place of the two electric field phase modulators.

Ninth Embodiment

[0157] In the fourth, fifth, sixth, and seventh embodiments, a
polarization rotation process which uses the polarization rotators 025-1,
025-2, and 025-3 or the electric field modulation processors 029-1 and
029-2 increases the number of modulation levels for electric field
signals entering the drive signal generators 015-1 and 015-2 and
deteriorates the signal waveform.

[0158] In the ninth embodiment, a limit is put on the above polarization
rotation process so that the number of modulation levels for electric
field signals entering the drive signal generators 015-1 and 015-2 does
not increase excessively. The limit on the polarization rotation process
should be such that the polarization rotation process is performed so as
to limit change in the electric field signals entering the drive signal
generators 015-1 and 015-2 as caused by the polarization rotation process
to a specific state.

[0159] For example, this is achieved by decreasing the least common
T3 of modulation cycle T1 of polarization modulation by the
polarization rotators 025-1, 025-2, and 025-3 or electric field
modulation by the electric field modulation processors 029-1 and 029-2
and modulation cycle T2 of electric field signals emitted from the
electric field mappers 014-1 and 014-2. For example, T1 is so
determined to make T3 smaller than a predetermined threshold.
Consequently, the number of modulation levels for electric field signals
entering the drive signal generators 015-1 and 015-2 is reduced to
T3/T2 of that in the case the polarization rotators 025-1,
025-2, and 025-3 or the electric field modulation processors 029-1 and
029-2 are not driven.

Tenth Embodiment

[0160] The following approach can be used in order to prevent the number
of modulation levels for electric field signals entering the drive signal
generators 015-1 and 015-2 in the seventh embodiment.

[0161] In the tenth embodiment, the electric field modulation controller
030 drives the electric field modulation processors 029-1 and 029-2 so
that the electric field symbols (symbol group or pattern) of electric
field signals entering the drive signal generators 015-1 and 015-2 become
electric field symbols of any of electric field signals entering the
electric field modulation processors 029-1 and 029-2.

[0162] For example, the 9QAM signal shown in FIG. 6B remains so even if it
is rotated 90 degrees. Therefore, if an electric field signal emitted
from the polarization multiplexer 023 is a 9QAM signal, the electric
field modulation processors 029-1 and 029-2 should modulate the phase of
the incoming signal at intervals of 90 degrees. In this case, the steps
from the electric field mappers 014-1 and 014-2 to the electric field
modulation processors 029-1 and 029-2 can be replaced by logical
operations of two data strings entering the electric field mappers 014-1
and 014-2.

Eleventh Embodiment

[0163] In order to prevent an increase in the number of modulation levels
for electric field signals entering the drive signal generators 015-1 and
015-2 in the seventh embodiment, the eleventh embodiment newly adds an
optical modulator to the orthogonally polarized signal generator 010 in
place of the electric field modulation processors 029-1 and 029-2.

[0164] FIG. 14 shows an example of the configuration of the eleventh
embodiment.

[0165] As compared with the seventh embodiment shown in FIG. 13, the
eleventh embodiments removes the electric field modulation processors
029-1 and 029-2 and the electric field modulation controller 030 for
driving them from the modulator driver 013 and inserts polarized light
modulators 031-1 and 031-2 at the output ends of the optical modulators
011-1 and 011-2 and adds a polarized light modulator driver 032. The
polarized light modulators 031-1 and 031-2 are driven by a polarized
light modulator driver 032 to modulate the amplitude ratio and phase
difference between two optical signals emitted from the optical
modulators 011-1 and 011-2.

[0166] In this embodiment, since the process of polarization rotation by
the modulator driver 013 is eliminated, the number of modulation levels
for electric field signals entering the drive signal generators 015-1 and
015-2 does not increase. Instead, the drive signals for the polarized
light modulators 031-1 and 032-2 which perform the polarization rotation
process generally become multileveled. However, the use of sinusoidal
waves with less inter-symbol interference as drive signals for the
polarized light modulators 031-1 and 031-2 reduces drive signal waveform
deterioration considerably.

[0167] An alternative approach is that without adding the polarized light
modulators 031-1 and 031-2, drive signals for driving the optical
modulators 011-1 and 011-2 and sinusoidal waves are superimposed on each
other and these signals are used as drive signals for the optical
modulators 011-1 and 011-2.

Twelfth Embodiment

[0168] In order to prevent waveform deterioration of electric field
signals entering the drive signal generators 015-1 and 015-2 in the
eighth embodiment, it is desirable that the number of modulation levels
should never increase.

[0169] Therefore, in the twelfth embodiment, two electric field signals
entering the drive signal generators 015-1 and 015-2 are alternated
cyclically. Consequently, the polarized waves of electric field signals
are cyclically changed without an increase in the number of modulation
levels for the two electric field signals entering the drive signal
generators 015-1 and 015-2.

[0170] FIG. 15 shows an example of the configuration of the twelfth
embodiment.

[0171] For example, the polarization multiplexing transmitter includes: an
orthogonal polarization multiplexing transmitter having an orthogonally
polarized signal generator 010 which has two optical modulators 011-1 and
011-2 for modulating the amplitudes and/or phases of two optical signals
and generates optical signals with two mutually different polarized waves
and a polarization multiplexer 012 which multiplexes the two optical
signals coming from the two optical modulators 011-1 and 011-2 and
outputs a single polarization multiplexed signal; and a modulator driver
013 which drives the two optical modulators 011-1 and 011-2. The
modulator driver 013 includes: a data string alternation device 033 which
alternates two incoming data strings cyclically and outputs two alternate
data strings; electric field mappers 014; and drive signal generators
015. The two electric field mappers 014-1 and 014-2, with a function to
convert data into electric fields uniquely, convert the two alternate
data strings into electric field signals respectively. The drive signal
generators 015-1 and 015-2 drive the optical modulators 011-1 and 011-2
so that the two electric field signals are consistent with the electric
fields of the optical signals modulated by the optical modulators 011-1
and 011-2.

[0172] An optical receiver which receives optical signals from the optical
transmitter according to this embodiment is expected to alternate the two
received polarized signals again. This alternation process in the
receiver should take place in synchronization with periodic data string
alternation by the data string alternation device 033. Various known
methods are available for this synchronization process. If the cycle and
pattern of data string alternation by the data string alternation device
033 are predetermined, the processing timing of the optical receiver
should be varied until a synchronization pattern such as a frame is
obtained from data strings demodulated by the optical receiver.
Especially, when the data string alternation device 033 alternates data
strings in synchronization with the electric field mappers 014-1 and
014-2 in their symbol cycles, the optical receiver is merely expected to
alternate the two polarized signals received by it in the symbol cycles.

[0173] In this embodiment, an electric field signal alternation device
which alternates two incoming electric field signals cyclically and
outputs two alternate electric field signals may be used in place of the
data string alternation device 033. If that is the case, the two electric
field mappers 014-1 and 014-2 convert the two data strings into electric
field signals respectively and the electric field signal alternation
device converts the two electric field signals into alternate electric
field signals before the two alternate electric field signals enter the
drive signal generators 015-1 and 015-2.

Thirteenth Embodiment

[0174] In an optical transmission system which uses a polarization
multiplexing transmitter (or optical polarization multiplexing
transmitter) according to any of the first to eleventh embodiments as an
optical transmitter to transmit optical signals from the transmitter to
an optical receiver, if the transmitter controls the polarization of
output optical signals so as to cancel a polarization fluctuation in the
transmission path from the transmitter to the receiver, the receiver need
not use a polarization tracer. Even if output polarized waves are limited
in the polarization multiplexing transmitter, it is possible to reduce
the burdens on the polarization tracer of the receiver and alleviate the
signal degradation caused by operation of the polarization tracer.

[0175] FIG. 16 shows an example of the configuration of the thirteenth
embodiment.

[0176] For example, the optical transmission system according to the
thirteenth embodiment includes: a polarization multiplexing transmitter
034 according to any of the first to eleventh embodiments; an optical
transmission path including an optical fiber transmission path 005 for
transmitting optical signals from the transmitter 034 and/or an optical
repeater (node) 006; an optical receiver 008 for receiving optical
signals propagated in the optical transmission path; and a polarization
tracer 007 for canceling a polarization fluctuation of an optical signal
entering the optical receiver 008. In addition, the optical transmission
system further includes: a transmission path polarization monitor 036
which detects polarization of an optical signal or the amount of
dependence thereon; a polarization management device 035 which checks for
a polarization fluctuation of the optical signal entering the optical
receiver 008 based on the result of detection by the transmission path
polarization monitor 036 and controls the polarized wave emitted from the
transmitter 034 so as to eliminate such polarization fluctuation; and a
system management device 037 which notifies the polarization management
device 035 of the polarization fluctuation detected by the transmission
path polarization monitor 036. If the optical receiver 008 is designed to
cancel a polarization fluctuation of an optical signal which it has
received, the polarization tracer 007 is omissible. If the transmission
path is short, the optical repeater 006 is also omissible.

[0177] An ordinary polarization monitor or any other means such as a
device which observes variations in reception sensitivity, waveform or
frequency due to polarization fluctuations may be used for the
transmission path polarization monitor 036.

[0178] One possible approach of polarization management is that, based on
the fact that the error rate of optical signals entering the optical
receiver 008 increases depending on their polarization fluctuations,
information on this error rate is transmitted to the polarization
management device 035 and the polarized waves emitted from the
polarization multiplexing transmitter 034 are feedback-controlled so as
to minimize the error rate.

Fourteenth Embodiment

[0179] In a wavelength multiplexing transmission system having
polarization multiplexing transmitters (or optical polarization
multiplexing transmitters) 034 according to any of the first to eleventh
embodiments and a polarization-uncontrolled transmitter 038 incapable of
controlling output polarized waves, comparison is made among the
transmission wavelengths of all the polarization multiplexing
transmitters 034 and the cycle and/or pattern of polarization scrambling
between polarization multiplexing transmitters 034 with adjacent
transmission wavelengths (wavelength patterns) is changed. Consequently,
the probability of coincidence of output polarization between
polarization multiplexing polarization multiplexing transmitters 034 with
adjacent transmission wavelengths decreases, thereby suppressing mutual
interaction between transmission signals from the transmitters 034 such
as four-wave mixing or cross-phase modulation. Even if the transmission
wavelength of the polarization-uncontrolled transmitter 038 is between
adjacent transmission wavelengths of two polarization multiplexing
transmitters 034, the probability of coincidence of output polarized
waves between the two transmitters 034 decreases, thereby suppressing the
influence of transmission signals from the two transmitters 034 on
transmission signals from the polarization-uncontrolled transmitter 038.
A known transmitter may be used for the polarization-uncontrolled
transmitter 038. The polarization-uncontrolled transmitter 038 is
omissible.

[0180] FIG. 17 shows an example of the configuration of the fourteenth
embodiment.

[0181] The optical transmission system according to this embodiment
includes: one or plural polarization multiplexing transmitters 034 which
control output polarized waves; one or plural polarization-uncontrolled
transmitters which do not control output polarized waves; a wavelength
multiplexer 039 which multiplexes transmission signals with different
wavelengths from these transmitters and outputs a wavelength-multiplexed
signal; an optical fiber transmission path 005 as an optical transmission
path for guiding the wavelength-multiplexed signal to an optical receiver
and an optical repeater (node) 006; a wavelength demultiplexer 040 which
demultiplexes the wavelength-multiplexed signal coming from the optical
transmission path into signals with different transmission wavelengths;
one or plural polarization tracing receivers 041 which receive
transmission signals from the polarization multiplexing transmitters 034
respectively; one or plural optical receivers 042 which receive
transmission signals from the polarization-uncontrolled transmitters 038
respectively; and a polarization management device (polarization
scrambling management device) 035 which manages output polarized waves
from the plural polarization multiplexing transmitters 034.

[0182] The polarization management device 035 manages the transmission
wavelengths and polarization of the plural polarization multiplexing
transmitters 034 and controls them so as to prevent coincidence of the
pattern and/or speed of polarization scrambling between polarization
multiplexing transmitters 034 with adjacent transmission wavelengths.

Configuration Examples of the Preferred Embodiments

[0183] An optical polarization multiplexing transmitter includes an
orthogonally polarized signal generator which multiplexes two optical
signals with mutually orthogonal polarized waves and two polarization
electric field modulators which modulate the amplitudes and/or phases of
the two optical signals respectively and the transmitter multiplexes an
arbitrary number of optical signals with arbitrary polarizations whose
amplitudes and/or phases are modulated and outputs a
polarization-multiplexed signal.

[0184] According to the first embodiment of the invention, for example,
there is provided an optical polarization multiplexing transmitter which
includes:

[0185] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two optical
signals with mutually orthogonal polarized waves, two optical modulators
which modulate the amplitudes and/or phases of the two optical signals,
and a polarization multiplexer which multiplexes two optical signals
emitted from the two optical modulators and outputs a single polarization
multiplexed signal;

[0186] an optical modulator driver having two electric field mappers, with
a function to convert data into electric fields uniquely, which convert
two data strings into electric field signals respectively and two drive
signal generators which drive the two optical modulators so that the two
electric field signals are consistent with the electric fields of two
optical signals emitted from the two optical modulators;

[0187] a polarization modulator which modulates the phase difference
between two mutually orthogonal polarized wave components on a
circumference of a circle having, in its plane, the center of the
Poincare sphere and a line perpendicular to a line connecting two
polarized waves from the orthogonal polarization multiplexing
transmitter; and a driver which drives the polarization modulator.

[0188] According to the second embodiment of the invention, for example,
there is provided an optical polarization multiplexing transmitter which
includes:

[0189] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two optical
signals with mutually orthogonal polarized waves respectively, two
optical modulators which modulate the amplitudes and/or phases of the two
optical signals respectively, two polarization modulators which modulate
the polarized waves of the two optical signals, and a polarization
multiplexer which multiplexes the two optical signals modulated by the
two polarization modulators and outputs a single polarization multiplexed
signal;

[0190] an optical modulator driver having two electric field mappers, with
a function to convert data into electric fields uniquely, which convert
two data strings into electric field signals respectively and two drive
signal generators which drive the two optical modulators so that the two
electric field signals are consistent with the electric fields of two
optical signals emitted from the two optical modulators; and

[0191] a polarization modulation driver which drives the two polarization
modulators so that the angle between the polarized waves of output
signals from the two polarization modulators is maintained constant and
the waves are modulated uniformly.

[0192] According to the third embodiment, for example, there is provided
an optical polarization multiplexing transmitter which includes:

[0193] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two optical
signals with mutually orthogonal polarized waves, two optical modulators
which modulate the amplitudes and/or phases of the two optical signals
respectively, and a polarization multiplexer which multiplexes two
optical signals modulated by the two optical modulators and outputs a
single polarization multiplexed signal; and

[0194] an optical modulator driver having plural electric field mappers,
with a function to convert data into electric fields uniquely, which
convert plural data strings into electric field signals respectively,
plural polarization mappers, with a function to convert an incoming
electric field into an arbitrary polarization having that electric field,
which convert the plural electric field signals from the plural electric
field mappers into polarized electric field signals with different
polarized waves respectively, a polarization multiplexer which
multiplexes the plural polarized electric field signals and generates a
single multiplexed polarized electric field signal, a polarization
demultiplexer which demultiplexes the multiplexed polarization electric
field signal into electric field signals with two polarized wave
components consistent with the two optical signals generated by the
orthogonally polarized signal generator, and two drive signal generators
which drive the two optical modulators so that the electric field signals
with two polarized wave components from the polarization demultiplexer
are consistent with the electric fields of the optical signals emitted
from the two optical modulators.

[0195] According to the fourth embodiment of the invention, for example,
there is provided an optical polarization multiplexing transmitter which
includes:

[0196] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two optical
signals with mutually orthogonal polarized waves, two optical modulators
which modulate the amplitudes and/or phases of the two optical signals
respectively, and a polarization multiplexer which multiplexes the two
optical signals modulated by the two optical modulators and outputs a
single polarization multiplexed signal; and

[0197] an optical modulator driver having plural electric field mappers,
with a function to convert data into electric fields uniquely, which
convert plural data strings into electric field signals respectively,
plural polarization mappers, with a function to convert an incoming
electric field into an arbitrary polarization having that electric field,
which convert the plural electric field signals from the plural electric
field mappers into polarized electric field signals with different
polarized waves respectively, plural polarization rotators which rotate
the polarized waves of the plural polarized electric field signals
respectively, a polarization rotation controller which controls the
plural polarization rotators, a polarization multiplexer which
multiplexes the polarized electric field signals emitted from the
polarization rotators into a multiplexed polarized electric field signal,
a polarization demultiplexer which demultiplexes the multiplexed
polarized electric field signal into electric field signals with two
polarized wave components consistent with the two optical signals
generated by the orthogonally polarized signal generator, and two drive
signal generators which drive the two optical modulators so that the
electric field signals with two polarized wave components from the
polarization demultiplexer are consistent with the electric fields of the
optical signals emitted from the two optical modulators.

[0198] Also, there is provided a polarization multiplexing transmitter
which multiplexes plural electric field signals with arbitrary
polarizations and outputs a multiplexed signal, in which the amplitudes
and/or phases of the plural electric fields are modulated respectively
and all polarized waves are uniformly modulated.

[0199] According to the fifth embodiment of the invention, for example,
there is provided a polarization multiplexing transmitter which includes:

[0200] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate the amplitudes and/or phases of the two
transmission electric fields respectively, and a polarization multiplexer
which multiplexes two transmission electric fields modulated by the two
electric field modulators and outputs a single polarization-multiplexed
signal; and

[0201] an electric field modulator driver having plural electric field
mappers, with a function to convert data into electric fields uniquely,
which convert plural data strings into electric field signals
respectively, polarization mappers, with a function to convert an
incoming electric field into an arbitrary polarization having that
electric field, which convert the plural electric field signals into
polarized electric field signals with different polarized waves, plural
polarization rotators which rotate the polarized waves of the plural
polarized electric field signals respectively, a polarization rotation
controller which synchronizes the plural polarization rotators and drives
the polarization rotators so as to rotate incoming polarized waves
uniformly and cyclically, a polarization multiplexer which multiplexes
the plural polarized electric field signals from the polarization
rotators into a multiplexed polarized electric field signal, a
polarization demultiplexer which demultiplexes the multiplexed polarized
electric field signal into electric field signals with two polarized wave
components consistent with the polarized waves of two transmission
electric fields generated by the orthogonally polarized signal generator,
and two drive signal generators which drive the two electric field
modulators so that the electric field signals with two polarized wave
components from the polarization demultiplexer are consistent with the
output electric fields from the two electric field modulators.

[0202] According to the sixth embodiment of the invention, for example,
there is provided a polarization multiplexing transmitter which includes:

[0203] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate the amplitudes and/or phases of the two
transmission electric fields respectively, and a polarization multiplexer
which multiplexes two transmission electric fields modulated by the two
electric field modulators and outputs a single polarization-multiplexed
signal; and

[0204] an electric field modulator driver having plural electric field
mappers, with a function to convert data into electric fields uniquely,
which convert plural data strings into electric field signals
respectively, polarization mappers, with a function to convert an
incoming electric field into an arbitrary polarization having that
electric field, which convert the plural electric field signals into
plural polarized electric field signals with different polarized waves, a
polarization multiplexer which multiplexes the plural polarized electric
field signals into a multiplexed polarized electric field signal, a
polarization rotator which rotates the multiplexed polarized electric
field signal, a polarization rotation controller which drives the
polarization rotator so as to rotate an incoming polarized wave
cyclically, a polarization demultiplexer which demultiplexes the
multiplexed polarized electric field signal from the polarization rotator
into electric field signals with two polarized wave components consistent
with the polarized waves of two transmission electric fields generated by
the orthogonally polarized signal generator, and two drive signal
generators which drive the two electric field modulators so that the
electric field signals with two polarized wave components from the
polarization demultiplexer are consistent with the output electric fields
from the two electric field modulators.

[0205] According to the seventh embodiment of the invention, for example,
there is provided a polarization multiplexing transmitter which includes:

[0206] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate the amplitudes and/or phases of the two
transmission electric fields, and a polarization multiplexer which
multiplexes two transmission electric fields modulated by the two
electric field modulators and outputs a single polarization-multiplexed
signal; and

[0207] an electric field modulator driver having plural electric field
mappers, with a function to convert data into electric fields uniquely,
which convert plural data strings into electric field signals
respectively, polarization mappers, with a function to convert an
incoming electric field into an arbitrary polarization having that
electric field, which convert the plural electric field signals into
plural polarized electric field signals with different polarized waves as
arbitrary polarizations on a circumference of a circle with an axis
connecting the polarized waves of the two transmission electric fields as
the axis of rotation on the Poincare sphere, a polarization multiplexer
which multiplexes the polarized electric field signals into a multiplexed
polarized electric field signal, a polarization demultiplexer which
demultiplexes the multiplexed polarized electric field signal into
electric field signals with two polarized wave components consistent with
the polarized waves of the two transmission electric fields, two electric
field modulation processors which modulate the amplitude ratio and/or
phase difference between two polarized wave components from the
polarization demultiplexer, an electric field modulation controller which
drives the electric field modulation processors, and two drive signal
generators which drive the two electric field modulators so that the
output electric fields from the two electric field modulation processors
are consistent with the two transmission electric fields modulated by the
electric field modulators respectively.

[0208] According to the ninth embodiment of the invention, for example,
there is provided a polarization multiplexing transmitter according to
any of the fourth to seventh embodiments in which the modulation cycle of
the polarization rotators as described in Claim 5, 7, or 8 or the
electric field modulation processors as described in Claim 7 is
determined so that the least common of the modulation cycle of the
polarization rotators as described in Claim 5, 7, or 8 or the electric
field modulation processors as described in Claim 7 and the modulation
cycle of electric field signals emitted from the above electric field
mappers is small.

[0209] According to the tenth embodiment of the invention, for example,
there is provided a polarization multiplexing transmitter according to
the seventh embodiment in which the two electric field modulators perform
modulation so that the electric field symbol group of incoming electric
field signals coincide with the electric field symbol group of outgoing
electric field signals.

[0210] According to the eleventh embodiment of the invention, for example,
there is provided a polarization multiplexing transmitter which includes:

[0211] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with mutually orthogonal polarized waves, two electric
field modulators which modulate the amplitudes and/or phases of the two
transmission electric fields, two polarization electric field modulators
which further modulate the amplitudes and/or phases of the two
transmission electric fields modulated by the two electric field
modulators, a polarization electric field modulator driver which drives
the two polarization electric field modulators so as to modulate the
amplitude ratio and/or phase difference between transmission electric
fields modulated by the two electric field modulators cyclically, and a
polarization multiplexer which multiplexes two transmission electric
fields from the two polarization electric field modulators and outputs a
single polarization-multiplexed signal; and

[0212] an electric field modulator driver having plural electric field
mappers, with a function to convert data into electric fields uniquely,
which convert plural data strings into electric field signals
respectively, polarization mappers, with a function to convert an
incoming electric field into an arbitrary polarization having that
electric field, which convert the plural electric field signals into
plural polarized electric field signals with different polarized waves as
arbitrary polarizations on a circumference of a circle with a line
connecting the polarized waves of the two transmission electric fields as
the axis of rotation on the Poincare sphere, a polarization multiplexer
which multiplexes the plural polarized electric field signals into a
multiplexed polarized electric field signal, a polarization demultiplexer
which demultiplexes the multiplexed polarized electric field signal into
electric field signals with two polarized wave components consistent with
the polarized waves of the two transmission electric fields, and two
drive signal generators which drive the two electric field modulators
respectively so that the electric field signals with two polarized wave
components from the polarization demultiplexer are consistent with the
transmission electric fields modulated by the two electric field
modulators respectively.

[0213] According to the twelfth embodiment of the invention, for example,
there is provided a polarization multiplexing transmitter which includes:

[0214] an orthogonal polarization multiplexing transmitter having an
orthogonally polarized signal generator which generates two transmission
electric fields with different polarized waves, two electric field
modulators which modulate the amplitudes and/or phases of the two
transmission electric fields, and a polarization multiplexer which
multiplexes two transmission electric fields emitted from the two
electric field modulators and outputs a single polarization-multiplexed
signal; and

[0215] an electric field modulator driver having a data string alternation
device which alternates two incoming data strings cyclically and outputs
two alternate data strings, two electric field mappers, with a function
to convert data into electric fields uniquely, which convert the two
alternate data strings into electric field signals respectively, and two
drive signal generators which drive the two electric field modulators so
that the two electric field signals from the electric field mappers are
consistent with the transmission electric fields modulated by the two
electric field modulators.

[0216] According to the thirteen embodiment of the invention, for example,
there is provided a transmission system which includes:

[0217] a polarization multiplexing transmitter according to any of the
first to eleventh embodiments which outputs a polarization-multiplexed
signal with a polarized wave so rotated as to cancel a polarization
fluctuation in a transmission path,

[0218] a transmission path polarization monitor which detects a
polarization fluctuation or its residual, or the amount of dependence
thereon, and

[0219] a polarization management device which drives the polarization
multiplexing transmitter based on the amount detected by the transmission
path polarization monitor.

[0220] According to the fourteenth embodiment of the invention, for
example, there is provided a wavelength multiplexing transmission system
which multiplexes transmission signals from polarization multiplexing
transmitters according to any of the first to eleventh embodiments and a
transmission signal from a transmitter which does not modulate the
polarized wave of a transmission signal, in which a polarization
scrambling management device manages and controls the polarization
scrambling pattern and/or speed of the polarization multiplexing
transmitters with different wavelength channels so that the polarization
scrambling patterns and/or speeds of polarization multiplexing
transmitters with adjacent wavelength channels among those transmitters
do not coincide with each other.

[0221] According to the above embodiments, it is possible to use an
orthogonal polarization multiplexing transmitter so as to generate a
polarization-scrambled optical signal without newly adding a polarization
modulator or polarization scrambler to an optical transmitter.
Particularly, it is also possible to generate polarization-scrambled
polarization multiplexed signal. In addition, since no additional
polarization modulator nor polarization scrambler is required, the
equipment cost and size can be reduced.

[0222] The polarization scrambling speed, cycle, timing and/or pattern of
an optical signal emitted from an optical transmitter can be as desired.
Particularly, the polarization scrambling speed can be a desired speed
not higher than the modulation speed of an optical signal.